System and method of correcting mask rule violations after optical proximity correction

ABSTRACT

A method is provided for correcting rule violating areas of a photomask using a digital representation of the photomask. The method includes identifying violating areas of the photomask from a digital representation of the photomask. The violating areas include areas that violate a minimum width rule and/or areas that violate a minimum space rule for the photomask. The violating areas are then manipulated for the purpose of eliminating the violating areas. They are manipulated differently based on whether the violating area lies inside a design shape of a layout pattern to be imaged using the photomask and/or whether the violating area lies outside the design shape.

BACKGROUND OF THE INVENTION

The present invention is related to photolithography. More specifically,the invention relates to a system and method for correcting ruleviolations of a digital representation of a photomask, especially afterthe digital representation has been corrected for optical proximity.

In the fabrication of integrated circuits, the process of converting adesired electrical circuit schematic into patterns to be imaged on anintegrated circuit (“IC” or “chip”) involves many steps. The electricalcircuit schematic is first converted into a desired chip layout having aset of patterns to be produced on a semiconductor chip. Such chip layouttypically includes conductor patterns, active semiconductor areapatterns and isolation area patterns at a lowest and most denselypatterned level of the chip. The chip layout also includes wiringpatterns at higher levels of the chip. These patterns are typicallyproduced on the chip by photolithographic imaging onto a photoresistlayer through several photomasks (hereinafter, “masks”), each maskhaving a set of mask patterns for producing the patterns in thephotoresist.

A photolithographic imaging system includes an illumination source, amask, and lenses for focusing an image produced by the mask onto aphotoresist layer on a substrate such as a semiconductor wafer.

The circuit layout is densest and the patterns are the smallest at thelowest (semiconductor) level of the wafer. The patterns are so smallthat the images produced by the photolithographic imaging system arenear the limit of resolution of the imaging system. Diffraction cancause the light passing through the openings of the mask to interfereconstructively and destructively.

Because of this, the smallest patterns on a wafer generally cannot beprinted directly by images of the same patterns on a mask. The imagingsystem simply cannot produce exactly the same image on the wafer thatappears on the mask. A phenomenon known as “photolithographic line endshortening” can occur, by which the ends of line patterns appear shorteras imaged in the photoresist than they do on the mask. The image canalso vary based on whether patterns are spaced at even spacings fromeach other or at irregular spacings.

Thus, the degree of change in the image from the mask patterns to thephotoresist patterns is a function of the proximity of patterns on themask in view of the size of the feature to be printed and the resolutionlimit of the imaging system.

Optical proximity correction (OPC) is a method used to correct for theabove effect to generate the patterns of a mask. The goal of OPC is togenerate a set of mask patterns that will result in the desired image ona wafer. Typically, a data set representing a layout pattern is providedto a computer system executing an OPC program. The OPC program predictschanges that would occur in the photoresist image if the mask containedthe same layout pattern. The layout pattern of the mask is then alteredby the OPC program in a way that is predicted to produce the correctimage on the wafer. All of these steps are performed according to aprogram executed on a computer with respect to a data set representingthe layout pattern.

For example, as shown in FIG. 1, a layout pattern includes a pluralityof design shapes including the shapes 10, 12, 14 and 16 and 18. Becauseof optical proximity, the design shapes may not be capable of beingprinted by a set of patterns of a mask having the same shapes.Accordingly, the layout pattern is processed according to an OPC programto generate a set of mask shapes that will result in a desired layoutpattern on the semiconductor wafer. FIG. 2 illustrates the correspondingmask shapes 200, 202, 204, 206 and 208 that result after OPC processing.The contours of the original design shapes 10, 12, 14, 16 and 18 areindicated in FIG. 2 by dotted line.

However, the result after OPC processing may still not be usable. Thisis because OPC processing is only concerned with the layout pattern tobe produced on the wafer, but not the patterns of the mask. Each masktype has a set of rules which define the limits of how patterns can beformed thereon. Pattern elements must conform to the mask rules. If thepattern elements do not conform to the rules, the mask cannot be made.After OPC processing, the data set representation of the mask patternsmay contain elements that cannot be fabricated on the mask. Suchelements are referred to as “mask rule violations”.

Therefore, a system and method is needed by which mask data representingOPC corrected mask shapes is corrected for mask rule violations.

It would be desirable to provide a system and method of generating maskpattern data following OPC that quickly arrives at final mask patterns.

It would further be desirable to provide a method for correcting maskrule violations in OPC corrected mask data that is expected to finishwithin a finite number of iterations.

It would further be desirable to provide a method for correcting maskrule violations in OPC corrected mask data that gradually reverses OPCcorrections to remove the mask rule violations.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a method is provided forcorrecting rule violating areas of a photomask using a digitalrepresentation of the photomask. The method includes identifyingviolating areas of the photomask from a digital representation of thephotomask. The violating areas include areas that violate a minimumwidth rule and/or areas that violate a minimum space rule for thephotomask. The violating areas are then manipulated for the purpose ofeliminating the violating areas. They are manipulated differently basedon whether the violating area lies inside a design shape of a layoutpattern to be imaged using the photomask and/or whether the violatingarea lies outside the design shape.

According to another aspect of the invention, a machine readable storagemedium is provided having a set of instructions recorded thereon forperforming a method for correcting rule violating areas of a photomaskusing a digital representation of the photomask. The method includesidentifying violating areas of the photomask from a digitalrepresentation of the photomask. The violating areas include areas thatviolate a minimum width rule and/or areas that violate a minimum spacerule for the photomask. The violating areas are then manipulated for thepurpose of eliminating the violating areas. They are manipulateddifferently based on whether the violating area lies inside a designshape of a layout pattern to be imaged using the photomask and/orwhether the violating area lies outside the design shape.

According to a particular aspect of the invention, the method furtherincludes manipulating the violating areas differently based on whetheran area violates a minimum width rule and whether an area violates aminimum space rule for the photomask.

According to an aspect of the invention, a violating area is manipulatedby enlarging an area that violates a minimum width rule when the arealies inside a design shape.

According to another aspect of the invention, a violating area ismanipulated by enlarging an area that violates a minimum space rule whenthe area lies outside a design shape.

According to yet another aspect of the invention, a violating area ismanipulated by removing an area that violates a minimum width rule whenthe area lies outside a design shape.

According to yet another aspect of the invention, a violating area ismanipulated by filling an area that violates a minimum space rule whenthe area lies inside a design shape.

According to yet another aspect of the invention, violating areas aremanipulated by enlarging an area that violates a minimum width rule whenthe area lies inside a design shape;

-   -   removing a violating area which violates a minimum width rule        when the violating area lies outside a design shape;    -   filling a violating area which violates a minimum space rule        when the violating area lies inside a design shape; and    -   enlarging a violating area which violates a minimum space rule        when the violating area lies outside a design shape.

According to still another aspect of the invention, the digitalrepresentation of the photomask is corrected for optical proximity priorto identifying the violating areas.

According to still another aspect of the invention, the violating areasinclude an area violating a minimum space rule between one filled areaof a mask shape and another filled area of the mask shape.

According to yet another aspect of the invention, violating areasinclude an area violating a minimum space rule between one mask shapeand another mask shape of the photomask.

According to another aspect of the invention, a system is operable tocorrect rule violations of a photomask using a digital representation ofthe photomask, the system being operable to identify violating areas ofthe photomask from a digital representation of the photomask, theviolating areas including at least one of areas violating a minimumwidth rule and areas violating a minimum space rule for the photomask,the system further being operable to manipulate each of the violatingareas differently based on the placement of the violating area relativeto a design shape of a layout pattern to be imaged using the photomask,the manipulation being for the purpose of eliminating the ruleviolations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating a set of design shapes of a chiplayout pattern to be printed on a wafer.

FIG. 2 is a plan view illustrating a set of OPC corrected mask shapescorresponding to the set of design shapes of FIG. 1.

FIG. 3 is a flow diagram illustrating a method embodiment of theinvention.

FIG. 4 is a plan view illustrating an OPC corrected mask shape prior toprocessing according to an embodiment of the invention.

FIG. 5 is a plan view illustrating a mask shape corrected by processingaccording to an embodiment of the invention.

FIG. 6 is a plan view illustrating a set of mask shapes corrected byprocessing according to an embodiment of the invention, the correctedmask shapes corresponding to the set of OPC corrected mask shapesillustrated in FIG. 2.

FIG. 7 illustrates a system embodiment of the invention.

DETAILED DESCRIPTION

The present invention provides a system and method of correcting maskrule violations in a digital representation of a photomask. The digitalrepresentation of the photomask is generated from a digitalrepresentation of a plurality of design shapes to be created on asemiconductor chip or wafer. Typically, optical proximity correction(OPC) is performed when generating the shapes of a mask from the designshapes. Through OPC, a set of mask shapes are created that are alteredfrom the design shapes to better enable the design shapes to be printedon the wafer. However, after OPC the mask shapes may violate mask rules.

In an embodiment of the invention, a process is carried out tomanipulate violating areas of the mask for the purpose of eliminatingthe violations of the violating areas. By carrying out the process, maskrule violations are gradually lessened, with the goal of eliminatingmask rule violations. In such way, particular OPC corrections thatviolate mask rules are gradually reversed until the mask rule violationsare satisfactorily removed from the mask. The process can be performediteratively multiple times with the purpose of eliminating all mask ruleviolations.

As illustrated in FIG. 3, a set of mask data following OPC are input tobe processed (S02). The mask data specifies a set of mask shapes thatare to be formed on the mask. Next, it is determined whether the maskshapes violate any mask rules. (S04). If the mask shapes do not violateany mask rules, then processing ends. (S05). However, if the mask shapesdo violate any mask rules, then further processing is performed tomanipulate the mask shapes for the purpose of eliminating mask ruleviolations.

Referring to FIG. 4, an original design shape 101 to be created on achip is indicated by the rectangular dotted outline. Such design shapeis illustrated merely by way of example, as the design shapes of a chiplayout pattern can be of many different shapes. The mask shape after OPCis illustrated at 100.

Mask rule violations are of two types. Minimum space violations occur inareas of the mask shape 100 where there is insufficient minimum spacebetween portions of the mask shape. Referring to FIG. 4, area 104 is anexample of a minimum space violation because the space 106 betweenfilled portions of the mask shape 100 is less than the required minimumspace for making the mask.

A minimum space violation can also occur where there is insufficientspace between one mask shape and another. As shown in FIG. 2, a minimumspace violation can occur between a portion of a mask shape and anothermask shape in proximity thereto. For example, a protruding portion 220of a mask shape 204 may violate a minimum space rule in relation to aprotruding portion 224 of another mask shape 200.

Referring again to FIG. 4, minimum width violations occur in areas wherea portion of a mask shape is smaller than minimum width. Areas 102, 112and 114 are examples of minimum width violations.

Referring again to FIG. 3, after it is determined that the OPC processedmask shapes violate a mask rule, processing is performed to compare themask shapes to the original design shapes of the wafer layout. (S06). Asshown in FIG. 4, a particular mask shape 100 can include an area 102that lies outside the original design shape 101. Alternatively, or inaddition thereto, the mask shape 100 can include an area 104 that liesinside the original design shape 101.

Beginning with step S07, processing then proceeds for every mask ruleviolation identified in step S04. In step S08, processing determineswhether a particular mask rule violation lies outside a design shape.(S08) If the answer is NO, i.e., the mask rule violation does not lieoutside the design shape, then processing proceeds to S10. Areas 104,108, 112 and 114 of mask shape 108 include areas that lie inside thedesign shape 101 indicated by the dotted line.

The manipulation of the mask rule violation is varied depending on thetype of violation. If the area violates a minimum space rule, then thearea, being inside a design shape, is filled. (S10, S12). Filling aminimum space violation that lies inside a design shape is consideredless likely to interfere with the OPC correction that was made ingenerating the mask shape 101. Since the minimum space violation isinside the design shape, filling it will generally not cause a minimumspace violation between the mask shape 101 and another mask shapeadjacent thereto (not shown).

As shown in FIG. 4, area 104 lies within the design shape 101 andviolates a minimum space rule. Area 108 violates a minimum space ruleand includes a portion which lies inside the design shape 101.

FIG. 5 illustrates the mask shape 500 after processing in accordancewith the embodiment described herein. As shown in FIG. 5, 504 and 508indicate former violating areas of the mask shape 500 that have beenfilled. With respect to the violating area 108, processing is performedin two parts: the processing of a portion lying within the design shape101 and the processing of a portion lying outside of the design shape101. As shown in FIG. 5, the portion of the minimum space violating area108 lying inside the design shape 101 has been filled. As described morefully below, the portion of the minimum space violating area 108 lyingoutside the design shape 101 is enlarged to become a wider space 508.

Processing is also performed based on whether the area violates aminimum width rule. If it violates a minimum width rule, then the area,being inside a design shape, is enlarged. (S14, S16). Enlarging aminimum width violation that lies inside a design shape is consideredunlikely to interfere with the OPC correction that was made ingenerating the mask shape 101. It is not likely to cause a minimum spaceviolation from the mask shape 500 to another shape adjacent thereto onthe mask (not shown) because the enlarged area lies inside a designshape 101.

As shown in FIG. 4, area 112 lies within the design shape 101 andviolate a minimum width rule. Area 114 violates a minimum width rule andincludes a portion which lies inside the design shape 101.

FIG. 5 illustrates the mask shape 500 after processing in accordancewith the embodiment described herein. As shown in FIG. 5, 512 and 514indicate former violating areas of the mask shape 500 that have beenenlarged. With respect to the violating area 114, processing isperformed in two parts: the processing of a portion lying within thedesign shape 101 and the processing of a portion lying outside of thedesign shape 101. As shown in FIG. 5, the portion of the minimum widthviolating area 114 that lies inside the design shape 101 has beenenlarged. As described more fully below, the portion of the minimumwidth violating area 114 lying outside the design shape 101 is removed.

If the violating area does not violate a minimum width rule, thenprocessing of the next violating area (S26) proceeds from step S07again. At step S08, processing determines whether the next violatingarea lies outside a design shape. (S08) This time, if the answer is YES,i.e., the violating area does lie outside the design shape 101, thenprocessing proceeds to S18. For example, areas 102, 108, 110 and 114either lie completely outside the design shape 101 or include areas thatlie outside the design shape 101.

Again, the manipulation of the violating area is varied depending on thetype of violation. If the area violates a minimum space rule, then thearea, being outside a design shape, is enlarged. (S18, S20). As shown inFIG. 4, area 110 lies outside the design shape 101 and violates aminimum space rule. As shown in FIG. 5, the open area 110 is enlarged asarea 510 to the point that it does not violate the minimum space rule.In the case of minimum space violating area 108, the portion of theviolating area that lies outside of the design shape 101 is enlarged, asshown at 508.

Enlarging a minimum space violation that lies outside a design shape isconsidered less likely to interfere with the OPC correction that wasmade in generating the mask shape. Enlarging the spacing of a minimumspace violating area will not cause the spacing between the mask shape500 and another mask shape (not shown) to violate a minimum space rule.However, it is possible that a minimum width violation might thereafteroccur in the contour of the mask shape 500 itself.

In step S22, processing is performed based on whether the area violatesa minimum width rule. If it violates a minimum width rule, then thearea, being outside a design shape, is removed. (S22, S24). Removing theviolating area will not cause a minimum space violation between the maskshape 101 and another shape of the mask adjacent thereto (not shown).102 and 114 indicate areas that violate a minimum width rule. Area 114includes a portion which lies outside the design shape 101. Only thatportion is removed, whereas the portion inside the design shape 101 isenlarged, as described above. As shown in FIG. 5, 502 and 514 indicateareas of the mask shape 500 corresponding to areas 102 and 114 afterprocessing.

If the particular area does not violate a minimum width rule, then theprocessing of the next violating area (S26) proceeds from step S07again. This process then continues for each violating area identifiedprior thereto in step S04 until all such violating areas have beenprocessed.

Then, when all violating areas have been processed, processing returnsto step S04 again to determine whether any area of the corrected maskstill violates a mask rule. For example, this could be true if theenlargement of a minimum width violating area caused another violation,such as a minimum space violation. Alternatively, the enlargement of aminimum space violating area could cause a minimum width violation tooccur. It is also possible that the filling of a minimum width violatingarea could cause a minimum space violation to occur.

If the answer is YES, that there is a mask rule violation, thenprocessing proceeds again from step S06 as described above. If theanswer is NO, and there are no mask rule violations, then processing iscompleted (S05). As a result of the processing, a data set representingOPC corrected shapes of the mask has now been generated that does notviolate the rules for fabricating the particular mask.

FIG. 6 is a top (plan) view of a set of shapes 600, 602, 604, 606 and608 of a corrected mask 650 corresponding to the shapes 200, 202, 204,206 and 208 of the mask 250, illustrating the changes resulting fromprocessing according to an embodiment of the invention. The differencesare as follow: areas 210 inside design shapes that violate minimum spacerules have been filled. In addition, areas 212 inside design shapes thatviolate minimum width rules have been enlarged.

As shown in FIG. 2, this results in the minimum width violating areas212 being enlarged to the point in which the minimum space violatingareas 210 surrounding them disappear. As for the minimum space violatingareas 210, the mask shapes 202 and 206 have been restored to theiroriginal contours coinciding with the original design shapes 12 and 16in those regions. In such case, the OPC corrections are said to havebeen “undone.”

On the other hand, for the violating areas that lie outside of thedesign shapes 10, 12, 14, 16, there is a different outcome. As shown inFIG. 6, processing removes a minimum width violating area 220 fromregion 621. The minimum space violating area 222 is enlarged, resultingin the larger space 622.

FIG. 7 illustrates a system according to an embodiment of the invention.As shown in FIG. 7, the system includes a central processing unit (CPU)710 provided with a memory 720. The CPU 710 may include a singleprocessor or a plurality of processors arranged to execute instructionsof a program in a parallel or semi-parallel manner. An input output(I/O) interface 730 is provided for inputting a program includinginstructions and data to the CPU 710 and for outputting the results ofexecuting a program. The I/O interface 730 preferably includes one ormore types of interfaces to removable digital storage media such as amagnetic disk, magneto-optic disk, read/write and/or read only opticaldisc, digital tape, removable disk drive and/or removable solid statememory such as a portable memory card. In addition to or in placethereof, the I/O interface preferably includes a network interface suchas a modem or network adapter card for permitting transfer ofinformation to and from a network. The I/O interface 730 may alsoinclude a display for outputting information to and/or inputtinginformation from a user. The I/O interface 730 may additionally includeone or more user interface devices such as a keyboard, mouse, speaker,joystick, scanner, printer, etc. and the like. To the extent that any ofthe above described types of removable storage media are present in theI/O interface, a program containing a set of instructions that is storedin such removable storage medium can be transferred as input 740 betweenthe I/O interface 730 and the CPU 710. In addition to the program, a setof data to be operated upon by the instructions is also input over theI/O interface 730. Once the program and the data set to be operated uponhave been loaded into the CPU 710, the CPU then executes the set ofinstructions of the program relative to the data and provides output 750to the I/O interface 730 connected thereto.

In an embodiment, a program containing instructions for performing amethod according to an embodiment of the invention is stored on one ormore removable storage media to be provided to the I/O interface 730 andloaded into the CPU 710. Alternatively, the program containing theinstructions is transferred from a storage medium such as a memory ofone or more computers or other storage devices of a network to a modem,network adapter or other device of the I/O interface 730 and furthertransferred therefrom to the CPU 710. After the CPU 710 receives andloads the program into memory, the program is then executed relative tothe set of data provided to the CPU 710. In such way, a method ofcorrecting mask rule violations is performed by a CPU executing aprogram relative to a set of OPC corrected data representing thepatterns of a mask.

As these and other variations and combinations of the features discussedabove can be utilized, the foregoing description of the preferredembodiments should be taken by way of illustration, rather than bylimitation of the invention, as defined by the claims.

1. A method of correcting rule violations of a photomask using a digitalrepresentation of the photomask, comprising: identifying violating areasof the photomask from a digital representation of the photomask, theviolating areas including at least one of areas violating a minimumwidth rule and areas violating a minimum space rule for said photomask;and manipulating each of the violating areas differently based on theplacement of the violating area relative to a design shape of a layoutpattern to be imaged using the photomask, said manipulating performedfor the purpose of eliminating the rule violations.
 2. The method ofclaim 1 further comprising manipulating each of the violating areasdifferently based on whether the area violates a minimum width rule andwhether the area violates a minimum space rule for said photomask. 3.The method of claim 2 wherein said manipulating includes enlarging anarea that violates said minimum width rule when said area lies inside adesign shape.
 4. The method of claim 2 wherein said manipulatingincludes enlarging an area that violates said minimum space rule whensaid area lies outside a design shape.
 5. The method of claim 2 whereinsaid manipulating includes removing an area that violates said minimumwidth rule when said area lies outside a design shape.
 6. The method ofclaim 2 wherein said manipulating includes filling an area that violatessaid minimum space rule when said area lies inside a design shape. 7.The method of claim 2 wherein said manipulating includes: enlarging anarea that violates said minimum width rule when said area lies inside adesign shape; removing an area which violates said minimum width rulewhen said area lies outside a design shape; filling an area whichviolates said minimum space rule when said area lies inside a designshape; and enlarging an area which violates said minimum space rule whensaid area lies outside a design shape.
 8. The method of claim 2 whereinsaid digital representation of said photomask is corrected for opticalproximity prior to said step of identifying said violating areas.
 9. Themethod of claim 8 wherein said violating areas include an area violatinga minimum space rule between a filled area of a mask shape and anotherfilled area of the mask shape.
 10. The method of claim 8 wherein saidviolating areas include an area violating a minimum space rule between amask shape and another mask shape of said photomask.
 11. A machinereadable storage medium having a set of instructions recorded thereonfor performing a method of correcting rule violations of a photomaskusing a digital representation of the photomask, said method comprising:identifying violating areas of the photomask from a digitalrepresentation of the photomask, the violating areas including at leastone of areas violating a minimum width rule and areas violating aminimum space rule for said photomask; and manipulating each of theviolating areas differently based on the placement of the violating arearelative to a design shape of a layout pattern to be imaged using thephotomask, said manipulating performed for the purpose of eliminatingthe rule violations.
 12. The machine readable storage medium of claim 11further comprising manipulating each of the violating areas differentlybased on whether the area violates a minimum width rule and whether thearea violates a minimum space rule for said photomask.
 13. The machinereadable storage medium of claim 12 wherein said manipulating includesenlarging an area that violates said minimum width rule when said arealies inside a design shape.
 14. The machine readable storage medium ofclaim 12 wherein said manipulating includes enlarging an area thatviolates said minimum space rule when said area lies outside a designshape.
 15. The machine readable storage medium of claim 12 wherein saidmanipulating includes removing an area that violates said minimum widthrule when said area lies outside a design shape.
 16. The machinereadable storage medium of claim 12 wherein said manipulating includesfilling an area that violates said minimum space rule when said arealies inside a design shape.
 17. The machine readable storage medium ofclaim 12 wherein said manipulating includes: enlarging an area thatviolates said minimum width rule when said area lies inside a designshape; removing an area which violates said minimum width rule when saidarea lies outside a design shape; filling an area which violates saidminimum space rule when said area lies inside a design shape; andenlarging an area which violates said minimum space rule when said arealies outside a design shape.
 18. The machine readable storage medium ofclaim 12 wherein said digital representation of said photomask iscorrected for optical proximity prior to said step of identifying saidviolating areas.
 19. The machine readable storage medium of claim 18wherein said violating areas include an area violating a minimum spacerule between a filled area of a mask shape and another filled area ofthe mask shape.
 20. The machine readable storage medium of claim 18wherein said violating areas include an area violating a minimum spacerule between a mask shape and another mask shape of said photomask. 21.A system operable to correct rule violations of a photomask using adigital representation of the photomask, said system being operable toidentify violating areas of the photomask from a digital representationof the photomask, the violating areas including at least one of areasviolating a minimum width rule and areas violating a minimum space rulefor said photomask, said system further being operable to manipulateeach of the violating areas differently based on the placement of theviolating area relative to a design shape of a layout pattern to beimaged using the photomask, said manipulation being for the purpose ofeliminating the rule violations.